Radio communication base station apparatus and radio communication method

ABSTRACT

A base station apparatus is provided, which includes a generator configured to generate a synchronization signal and a transmitter configured to transmit the generated synchronization signal. The generator is configured to generate a synchronization signal to be mapped on a subcarrier included in one of a plurality of frequency resource candidates that are separated by an interval, which is a common multiple of a determined frequency spacing and a subcarrier spacing between contiguous subcarriers, wherein the subcarrier spacing does not have a value that is a divisor of the determined frequency spacing.

This is a continuation application of application Ser. No. 12/160,483filed Jul. 10, 2008, which is a national stage of PCT/JP2007/050830filed Jan. 19, 2007, which is based on Japanese Application No.2006-012436 filed Jan. 20, 2006, the entire contents of each which areincorporated by reference herein.

TECHNICAL FIELD

The present invention relates to a radio communication base stationapparatus and a radio communication method.

BACKGROUND ART

In the mobile communication system, a radio communication mobile stationapparatus (hereinafter “mobile station”) performs a cell search uponpower activation or upon handover. This cell search is performed usingan SCH (synchronization channel). The SCH is a shared channel in thedownlink direction and is comprised of a P-SCH (primary synchronizationchannel) and an S-SCH (secondary synchronization channel). P-SCH datacontains a sequence which is common in all cells and which is used forthe timing synchronization upon the cell search. Further, S-SCH datacontains cell-specific transmission parameters such as scrambling codeinformation. In a cell search upon power activation or upon handover,each mobile station finds the timing synchronization by receiving P-SCHdata and acquires transmission parameters that differ between cells byreceiving S-SCH data. By this means, each mobile station can startcommunicating with radio communication base station apparatuses(hereinafter “base stations”). Therefore, each mobile station needs todetect SCH data upon power activation or upon handover.

Further, according to the FFD scheme standard proposed by 3GPP,frequencies for setting carriers are arranged at 200 kHz intervals in a60 MHz frequency bandwidth (see Patent Document 1). Therefore, accordingto this standard, the frequency interval the mobile station performs acell search is 200 kHz. That is, the mobile station performs a cellsearch every 200 kHz.

Further, to simplify the design of the communication system, the SCH isgenerally set in the center frequency of the frequency bandwidth inwhich a mobile station can perform communication.

By the way, in recent years, in mobile communication, various kinds ofinformation such as images and data as well as speech are subjected totransmission. With this trend, it is expected that demands furtherincrease for high reliability and high speed transmission. However, whenhigh speed transmission is performed in mobile communication, theinfluence of delayed waves by multipath is not negligible, andtransmission performance degrades due to frequency selective fading.

Multicarrier communication such as OFDM (Orthogonal Frequency DivisionMultiplexing) has attracted attention as one of counter techniques forfrequency selective fading. Multicarrier communication refers to atechnique of performing high speed transmission by transmitting datausing a plurality of subcarriers of transmission rates suppressed tosuch an extent that frequency selective fading does not occur.Particularly, with the OFDM scheme, frequencies of a plurality ofsubcarriers where data is arranged are orthogonal to each other, therebyenabling the maximum frequency efficiency in multicarrier communicationand implementation with relatively simple hardware configuration. Bythis means, the OFDM scheme has attracted attention as a communicationmethod applied to cellular scheme mobile communication and is variouslystudied. In the communication system employing the OFDM scheme, theinterval between adjacent subcarriers (subcarrier intervals) in aplurality of subcarriers, is set according to the coherence bandwidth(the frequency bandwidth in which channel fluctuation is the same) ofthis communication system.

Further, at present, according to the LTE standardization of 3GPP, inthe mobile communication system with the OFDM scheme, allowing aplurality of mobile stations communicating in respective bandwidths, toperform communication in the system, is studied. This mobilecommunication system can be referred to as a “scalable bandwidthcommunication system.”

For example, assuming the scalable bandwidth communication system havinga 20 MHz operating frequency bandwidth, if the 20 MHz operatingfrequency bandwidth is equally divided per 5 MHz frequency bandwidthinto four frequency bands FB1 FB2, FB3 and FB4, it is possible to usemobile stations having 5 MHz, 10 MHz or 20 MHz communication capacitiesat the same time. In the following explanation, out of a plurality ofmobile stations that are available, the mobile station having theminimum communication capacity is referred to as the “minimum capacitymobile station,” and the mobile station having the maximum communicationcapacity is referred to as the “maximum capacity mobile station.”Therefore, in this case, the mobile station having the 5 MHzcommunication capacity is the minimum capacity mobile station and themobile station having the 20 MHz communication capacity is the maximumcapacity mobile station.

Further, for example, assuming the scalable bandwidth communicationsystem with a 4.2 MHz operating frequency bandwidth, if the 4.2 MHzoperating bandwidth is divided per 2.1 MHz frequency bandwidth into twobandwidths FB1 and FB2, it is possible to use a mobile station having a2.1 MHz communication capacity and a mobile station having a 4.2 MHzcommunication capacity at the same time. Therefore, in the above, themobile station having a 2.1 MHz communication capacity is the minimumcapacity mobile station and the mobile station having a 4.2 MHzcommunication capacity is the maximum capacity mobile station. A mobilestation having a 2.1 MHz communication capacity is referred to as a “2.1MHz mobile station,” and a mobile station having a 4.2 MHz communicationcapacity is referred to as a “4.2 MHz mobile station.” In this scalablebandwidth communication system, the 2.1 MHz mobile station is assigned a2.1 MHz frequency bandwidth out of the 4.2 MHz frequency bandwidth andperforms communication. That is, the 2.1 MHz mobile station is assignedone of FB1 and FB2 and performs communication. Further, the 4.2 MHzmobile station can perform high speed communication using the entire 4.2MHz operating frequency bandwidth. Here, as described above, thefrequency bandwidth, in which the maximum capacity mobile station canperform communication, generally matches the frequency bandwidth where ascalable bandwidth communication system is operated (in this case, 4.2MHz).

-   Patent Document 1: Japanese Patent Application Laid-Open No.    2003-60551

DISCLOSURE OF INVENTION Problems to be Solved by the Invention

Here, assuming that the OFDM scheme is applied to a scalable bandwidthcommunication system as shown in FIG. 1, if the SCH is set in the centerfrequency of the frequency bandwidth in which a mobile station canperform communication, the mobile station that performs a cell search atpredetermined cell search intervals as above may not be able to performa cell search. For example, assume that the SCH for the 4.2 MHz mobilestation is set in the center frequency f_(c1) of a 4.2 MHz frequencybandwidth, and the SCH for the 2.1 MHz mobile station is set in thecenter frequency f_(c2) of a 2.1 MHz frequency bandwidth. Further,assume that the subcarrier interval is set 150 kHz according to thecoherence bandwidth in the communication system. Further, assume thatthe cell search interval is 200 kHz as above. Here, if the centerfrequency f_(c1) is set a frequency that is an integral multiple of the200 kHz cell search interval, although the 4.2 MHz mobile station candetect the SCH, the 150 kHz subcarrier interval makes the 2.1 MHz mobilestation unable to detect the SCH and perform a cell search. By contrast,if the center frequency f_(c2) is set the frequency that is an integralmultiple of the 200 kHz cell search interval, although the 2.1 MHzmobile station can detect the SCH, the 150 kHz subcarrier interval makesthe 4.2 MHz mobile station unable to detect the SCH and perform a cellsearch. Thus, if the OFDM scheme is applied to a scalable bandwidthcommunication system where there are a plurality of mobile stationscommunicating in respective bandwidths, there are mobile stations thatcannot perform a cell search depending on the relationship between thesubcarrier interval and the cell search interval.

One possible solution to this problem is to decide the subcarrierinterval according to the cell search interval. To be more specific,making the cell subcarrier a divisor of the search interval, ispossible. However, with this solution, it is not always possible to setan optimal subcarrier interval according to the coherence bandwidth,and, consequently, throughput degradation and error rate performancedegradation may be caused.

It is therefore an object of the present invention to provide, in thescalable bandwidth communication system adopting the OFDM scheme, a basestation and radio communication scheme for enabling all of a pluralityof mobile stations communicating in respective frequency bandwidths, toperform a cell search.

Means for Solving the Problem

The radio communication base station of the present invention thattransmits a multicarrier signal comprised of a plurality of subcarriers,employs a configuration having: a setting section that sets one of theplurality of subcarriers as a first subcarrier for transmitting asynchronization channel signal; a generating section that generates themulticarrier signal by mapping the synchronization channel signal on thefirst subcarrier; and a transmitting section that transmits themulticarrier signal, and the configuration in which, among the pluralityof subcarriers, the setting section sets, as the first subcarrier, oneof subcarriers having frequencies of common multiples of the subcarrierinterval between the plurality of subcarriers and a frequency intervalin which a radio communication mobile station performs a cell search.

Advantageous Effect of the Invention

According to the present invention, all of a plurality of mobilestations communicating in respective frequency bandwidths can perform acell search in the scalable bandwidth communication system adopting, forexample, the OFDM scheme.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates a scalable bandwidth communication system adopting anOFDM scheme;

FIG. 2 is a block diagram showing a configuration of a base stationaccording to an embodiment of the present invention;

FIG. 3 illustrates an SCH setting example according to an embodiment ofthe present invention (setting example 1);

FIG. 4 illustrates an SCH setting example according to an embodiment ofthe present invention (setting example 2); and

FIG. 5 illustrates an SCH setting example according to an embodiment ofthe present invention (setting example 3).

BEST MODE FOR CARRYING OUT THE INVENTION

An embodiment of the present invention will be explained below in detailwith reference to the accompanying drawings. Here, in the followingexplanation, although the OFDM scheme is explained as an example of amulticarrier communication scheme, the present invention is not limitedto the OFDM scheme.

FIG. 2 illustrates the configuration of base station 100.

Encoding section 101 encodes SCH data.

Modulating section 102 modulates the encoded SCH data.

Encoding section 103 encodes user data.

Modulating section 104 modulates the encoded user data.

Subcarrier setting section 105 sets one of a plurality of subcarriersforming an OFDM symbol, which is a multicarrier signal, as thesubcarrier for transmitting SCH data (SCH subcarrier). This subcarriersetting will be described later in detail.

According to the setting in subcarrier setting section 105, IFFT section106 maps SCH data and user data on the multiple subcarriers above andperforms an IFFT (Inverse Fast Fourier Transform), thereby generating anOFDM symbol. In this case, out of the multiple subcarriers above, IFFTsection 106 maps SCH data on the subcarrier set in subcarrier settingsection 105.

The OFDM symbol generated as above is attached a cyclic prefix (CP) inCP attaching section 107, subjected to predetermined radio processingsuch as up-conversion in radio transmitting section 108, and transmittedby radio from antenna 109 to mobile stations.

Here, in the OFDM scheme, to prevent intersymbol interference (“ISI”),the tail end of each ODFM symbol is attached to the head of that OFDMsymbol as a CP. By this means, a mobile station which is the receivingside can prevent ISI as long as the delay time of delay weaves stayswithin the range of the CP time period.

Next, subcarrier setting in subcarrier setting section 105 will beexplained in detail. Setting examples 1 to 3 (FIGS. 3 to 5) will beexplained below. Here, as above, the scalable bandwidth communicationsystem is assumed where the operating frequency bandwidth is 4.2 MHz andthere are a 2.1 MHz mobile station and a 4.2 MHz mobile station.Further, assume that the subcarrier interval is set 150 kHz as above.Further, assume that the cell search interval is set 200 kHz as above.

Setting Example 1 FIG. 3

Subcarrier setting section 105 sets, as the SCH subcarrier, one ofsubcarriers having frequencies of common multiples of the subcarrierinterval and the cell search interval, among the above multiplesubcarriers. That is, subcarrier setting section 105 sets, as the SCHsubcarrier, one of subcarriers having frequencies of common multiples ofthe 150 kHz subcarrier interval and the 200 kHz cell search interval(600 kHz×n, where n is a natural number). To be more specific, forexample, as shown in FIG. 3, subcarrier setting section 105 sets, as theSCH subcarrier, subcarrier f₁₂ having a frequency 1.8 MHz greater thanthe center frequency f_(c1) of 4.2 MHz. Therefore, for example, if thecenter frequency f_(c1) is set 2 GHz, the frequency of subcarrier f₁₂ is2001.8 MHz, which is an integral multiple of the 200 kHz cell searchinterval.

Thus, according to the present setting example, the SCH can be set in asubcarrier having an integral multiple frequency of the cell searchinterval among a plurality of subcarriers having predeterminedsubcarrier intervals, so that both the 2.1 MHz mobile station and the4.2 MHz mobile station having the same cell search interval can detectthe SCH and perform a cell search.

As shown in FIG. 3, the 2.1 MHz mobile station needs not change thecommunication frequency band between the time during cell search andother times such as during normal reception, and, consequently, is ableto receive all user data that can be received during normal receptionduring cell search, so that it is possible to prevent throughputdegradation according to the change of the communication frequency band.Further, the 2.1 MHz mobile station needs not change the communicationfrequency band between the time during cell search and the time duringnormal reception, that is, the 2.1 MHz mobile station needs not switchthe center frequency in radio reception between the time during cellsearch and the time during normal reception, so that it is possible toease control upon a cell search and reduce power consumption of mobilestations.

Setting Example 2 FIG. 4

In the above multiple subcarriers, subcarrier setting section 105 sets,as the SCH subcarrier, the subcarrier that is closest to the centerfrequency of the frequency bandwidth in which a mobile station canperform communication, out of subcarriers having frequencies of commonmultiples of the subcarrier interval and the cell search interval.

To be more specific, for example, as shown in FIG. 4, subcarrier settingsection 105 sets, as the SCH subcarrier, subcarrier f_(c1) that isclosest to the center frequency f_(c2) of 2.1 MHz, out of subcarriershaving 600×n frequencies (n is a natural number). That is, with thepresent setting example, among the subcarriers having 600 kHz×nfrequencies (n is a natural number), the SCH subcarrier is set in thesubcarrier that is closest to the center frequency of a frequencybandwidth which is narrower than the operating frequency bandwidth ofthe scalable bandwidth communication system and in which mobile stationsother than the maximum capacity mobile stations can performcommunication. Specifically, with the present setting example, all of aplurality of mobile stations communicating in respective frequencybandwidths need not switch the center frequency in radio communicationbetween the time during cell search and the time during normalreception, so that it is desirable to set, as the SCH subcarrier, thesubcarrier that is closest to the center frequency of the frequencybandwidth in which the maximum capacity mobile station can performcommunication.

Therefore, according to the present setting example, as in settingexample 1, the SCH can be set in a subcarrier having an integralmultiple frequency of the cell search interval among a plurality ofsubcarriers having predetermined subcarrier intervals, so that the 2.1MHz mobile station and the 4.2 MHz mobile station having the same cellsearch interval can detect the SCH and perform a cell search.

Further, according to the present setting example, as in setting example1, the 2.1 MHz mobile station needs not change the communicationfrequency band between the time during cell search and the other timessuch as during normal reception, and, consequently, is able to receiveall user data that can be received during normal reception during cellsearch, so that it is possible to prevent throughput degradationaccording to the change of the communication frequency band. Further,the 2.1 MHz mobile station needs not change the communication frequencyband between the time during cell search and the time during normalreception, that is, the 2.1 MHz mobile station needs not switch thecenter frequency in radio reception between the time during cell searchand the time during normal reception, so that it is possible to easecontrol during cell search and reduce power consumption of mobilestations.

Further, according to the present setting example, as shown in FIG. 4,the communication frequency bandwidth during cell search can be setnarrower than the communication frequency bandwidth during normalreception, so that it is possible to make the sampling rate for A/Dconversion during cell search smaller than the sampling rate for A/Dconversion during normal reception, and, as a result, further reducepower consumption of mobile stations.

Setting Example 3 FIG. 5

In the above multiple subcarriers, subcarrier setting section 105 sets,as the SCH subcarrier, the subcarrier that is closest to the centerfrequency of the operating frequency bandwidth of the communicationsystem, among the subcarriers having frequencies of common multiples ofthe subcarrier interval and the cell search interval.

To be more specific, for example, as shown in FIG. 5, subcarrier settingsection 105 sets, as the SCH subcarrier, subcarrier f₄ that is closestto the center frequency f_(c1) of 4.2 MHz, among the subcarriers having600 kHz×n frequencies (n is a natural number). That is, with the presentsetting example, the SCH subcarrier is set the subcarrier that isclosest to the center frequency of the operating frequency bandwidth ofthe scalable bandwidth communication system, among the subcarriershaving 600 kHz×n frequencies (n is a natural number). In other words,with the present setting example, the SCH subcarrier is set thesubcarrier that is closest to the center frequency of the frequencybandwidth in which the maximum capacity mobile station can performcommunication, among the subcarriers having 600 kHz×n frequencies (n isa natural number).

Therefore, according to the present setting example, as in settingexample 1, the SCH can be set in a subcarrier having an integralmultiple frequency of the cell search interval among a plurality ofsubcarriers having predetermined subcarrier intervals, so that both the2.1 MHz mobile station and the 4.2 MHz mobile station having the samethe cell search interval can detect the SCH and perform a cell search.

Further, according to the present setting example, as shown in FIG. 5,the communication frequency bandwidth during cell search can be setnarrower than the communication frequency bandwidth during normalreception, so that it is possible to make the sampling rate for A/Dconversion during cell search smaller than the sampling rate for A/Dconversion during normal reception, and, as a result, further reducepower consumption of mobile stations.

Here, compared to the present setting example to setting example 2,while the SCH subcarrier is set the subcarrier having the frequency thatis closest to the center frequency f_(c2) in setting example 2, the SCHsubcarrier is set the subcarrier having the frequency that is closest tothe center frequency f_(c1) in the present setting example. By thismeans, the present setting example is particularly useful when there aremore maximum capacity mobile stations than the other mobile stations,and setting example 2 is useful in the opposite case.

Thus, according to the present embodiment, in the scalable bandwidthcommunication system adopting a multicarrier communication scheme suchas the OFDM scheme, all of a plurality of mobile stations communicatingin respective frequency bandwidths are able to perform a cell search.

One embodiment of the present invention has been described above.

Here, the present invention is applicable to other shared channels thanthe SCH channel. As a shared channel other than the SCH, for example,there are BCH (broadcast channel) and SCCH (shared control channel).

Further, a base station may be referred to as “Node B,” a mobile stationas “UE,” a subcarrier as a “tone,” a cyclic prefix as a “guardinterval.”

Although a case has been described with the above embodiments as anexample where the present invention is implemented with hardware, thepresent invention can be implemented with software.

Furthermore, each function block employed in the description of each ofthe aforementioned embodiments may typically be implemented as an LSIconstituted by an integrated circuit. These may be individual chips orpartially or totally contained on a single chip.

“LSI” is adopted here but this may also be referred to as “IC,” “systemLSI,” “super LSI,” or “ultra LSI” depending on differing extents ofintegration.

Further, the method of circuit integration is not limited to LSI's, andimplementation using dedicated circuitry or general purpose processorsis also possible. After LSI manufacture, utilization of an FPGA (FieldProgrammable Gate Array) or a reconfigurable processor where connectionsand settings of circuit cells in an LSI can be reconfigured is alsopossible.

Further, if integrated circuit technology comes out to replace LSI's asa result of the advancement of semiconductor technology or a derivativeother technology, it is naturally also possible to carry out functionblock integration using this technology. Application of biotechnology isalso possible.

The disclosure of Japanese Patent Application No. 2006-012436, filed onJan. 20, 2006, including the specification, drawings and abstract, isincorporated herein by reference in its entirety.

INDUSTRIAL APPLICABILITY

The present invention is suitable to, for example, OFDM mobilecommunication systems.

1. A base station apparatus comprising: a generator configured togenerate a synchronization signal to be mapped on a subcarrier includedin one of a plurality of frequency resource candidates that areseparated by an interval, which is a common multiple of a determinedfrequency spacing and a subcarrier spacing between contiguoussubcarriers, wherein the subcarrier spacing does not have a value thatis a divisor of the determined frequency spacing; and a transmitterconfigured to transmit the generated synchronization signal.
 2. The basestation apparatus according to claim 1, wherein the determined frequencyspacing is a spacing between frequencies at which carriers can be set.3. The base station apparatus according to claim 1, wherein thedetermined frequency spacing is a frequency spacing for performing acell search.
 4. The base station apparatus according to claim 1, whereinthe subcarrier on which the synchronization signal is mapped is around acenter frequency of said one of the plurality of frequency resourcecandidates that includes said subcarrier.
 5. The base station apparatusaccording to claim 1, wherein the subcarrier on which thesynchronization signal is mapped is a subcarrier that is closest to acenter frequency of a communication frequency bandwidth.
 6. The basestation apparatus according to the claim 1, wherein the determinedfrequency interval is a channel raster of the base station apparatus. 7.A transmission method performed by a base station apparatus, comprising:generating a synchronization signal to be mapped on a subcarrierincluded in one of a plurality of frequency resource candidates that areseparated by an interval, which is a common multiple of a determinedfrequency spacing and a subcarrier spacing between contiguoussubcarriers, wherein the subcarrier spacing does not have a value thatis a divisor of the determined frequency spacing; and transmitting thegenerated synchronization signal.
 8. An integrated circuit for awireless communication apparatus, comprising: a generation section thatgenerates a synchronization signal to be mapped on a subcarrier includedin one of a plurality of frequency resource candidates that areseparated by an interval, which is a common multiple of a determinedfrequency spacing and a subcarrier spacing between contiguoussubcarriers, wherein the subcarrier spacing does not have a value thatis a divisor of the determined frequency spacing; and a transmissionsection that transmits the generated synchronization signal.